The use of material compound wafers, for example silicon on insulator (“SOI”) or silicon on quartz (“SOQ”) wafers, has become more and more widespread in the manufacturing process of modern electronic, optoelectronic, or micromechanical devices. Therein, use is made of the different physical or chemical properties of at least two materials that are attached together in such a structure. For example, in the case of an SOQ wafer, a thin silicon layer is provided on a quartz plate so that in an optoelectronic application, electronic circuits can be developed in and on the semiconducting silicon layer and at the same time, the quartz substrate can be used as the cover of the electronic device through which light can enter or exit the device.
To make such material compound wafers not only technologically interesting, but also economically viable, it becomes necessary to provide a manufacturing process of these material compound wafers that is capable of producing high quality material compound wafers, like the SOQ wafers, which are also economically viable.
In the case of SOI wafers, one example of such a suitable manufacturing process is based on SMART CUT® technology, which comprises the following process steps: implanting atomic species in a source substrate thereby forming an in-depth weakened layer corresponding to a predetermined splitting area, then bonding the source substrate to a handle substrate to form an assembly, and finally performing a splitting or detachment step, which is achieved by providing thermal or mechanical energy to the assembly, whereby the splitting occurs along the weakened area or implanted ions. The result of this process is that a thin layer from the source substrate is transferred onto the handle substrate. Usually a final treatment takes place to obtain a surface quality suitable for the desired application.
It is known that with raising temperature, such as by heating, the implanted layer becomes weaker and weaker, wherein the strength of the bonding interface gets stronger so that the transfer of the thin layer onto the handle substrate becomes possible after a certain temperature is achieved. Cho, Y., Cheung, N. W. “Low Temperature Si Layer Transfer by Direct Bonding and Mechanical Ion Cut” Appl. Phys. Lett. vol. 83, no. 18 (Nov. 3, 2003). In this reference, forces were measured with a probe destructive method by introducing a razor blade into the bonded structure to achieve a detachment either at the bonding interface or at the implanted layer. From this, an estimate was made of the necessary surface energy, which is related to the strength of the bonding interface and the implanted layer.
In the case of hetero-structures, and more particularly in the case of hetero-structures composed of materials having different characteristics, e.g., different thermal expansion coefficients, the above mentioned splitting step is usually carried out in two separate steps. First, an annealing step is used to further weaken the predetermined splitting area, and then a detachment step is used during which either thermal or mechanical energy is provided to the weakened area to achieve the actual detachment of the two structures. This two-step process is necessary due to the different thermal expansion coefficients of the materials, and the mechanical stresses that occur in the assembly. These factors ultimately lead to a reduction of production yield, as the hetero-structure may break due the internal stress at an elevated temperature.
However, with the economical pressure on the prices of wafers being extremely high, the production yield for the production of material compound wafers, particularly for heterogeneous material compound wafers, is not sufficient with the above described method.
Thus, the present invention now provides a manufacturing method and apparatus for facilitating splitting of such material compound wafers, in particular heterogeneous material compound wafers, and for which the production yield and quality of the final product is further enhanced.